Genome length strongly correlates with cell size; even more than cell complexity, number of protein-coding genes, etc. This was extremely surprising to me.
Initially, I expected that genome size would correlate with protein-coding genes. Although there is a linear relationship in prokaryotes (which have compact genomes with fewer regulatory elements), there is a logarithmic relationship in eukaryotes. A eukaryote called Edhazardia aedis, for example, has a genome with ~51 million nucleotides that only encodes 4,200 proteins. Some bacteria have genomes that are about an order-of-magnitude smaller in length, yet encode more proteins than this!
And what of cell complexity? Surprisingly, there is no relationship between genome length and complexity. Eukaryote genomes range in size by 200,000-fold. There are amoebas, salamanders, and small plants with genomes much larger than our own. An onion’s genome is five times larger than a human’s.
The closest correlation — and one that scales across kingdoms of life — is between genome length and cell size. Many papers on this subject have been written by T. Ryan Gregory, a Canadian biologist, who has collected thousands of examples of genome sizes and cell sizes. Gregory also maintains a database on genome sizes across the tree of life, at genomesize[dot]com.
In a 2007 paper, Gregory plotted this relationship for red blood cells taken from various organisms, such as fishes, amphibians, reptiles, and birds. (Red blood cells were selected so that each “type” of cell would be standardized across the organisms.) See chart #1 below.
Many recent papers continue to show the same relationship. I downloaded raw data from a 2023 paper, for example, that lists genome sizes and cell volumes for thousands of bacteria and eukaryotes. 53 organisms in this dataset have both a recorded cell volume *and* genome size, and those points are plotted in the second chart below.
The question is why this relationship exists at all. What does genome size have to do with cell size?
Many biologists argue for some kind of physical scaling. The size of a cell’s nucleus corresponds closely with its overall size, and most cells keep their “nuclear-to-cytoplasmic volume ratio” at a constant level. The more DNA a cell has, then, the more space it occupies, and the larger its nucleus (and overall cell size) must be to maintain this ratio.
This explanation is unsatisfying. For one, bacteria don’t have a nucleus, so why does this scaling apply to them? And second, the genome typically occupies less than 1% of the total nucleus volume, so why would a larger genome lead to a bigger nucleus mechanistically? There is plenty of space in there!
(Sidebar: A tiny fern from a South Pacific island has the world’s largest genome: 160.45 billion bases, more than 50-times larger than a human genome. If stretched out, this genome would be longer than the Statue of Liberty is tall; and yet, it occupies only a small portion of the fern’s nucleus.)
The reality seems to be that biologists don’t really understand (to a satisfying degree) why this relationship is true. Simple questions in biology often yield exceptionally complex answers.
